2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2014 Marco Costalba, Joona Kiiski, Tord Romstad
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
38 #include "syzygy/tbprobe.h"
43 volatile SignalsType Signals;
45 std::vector<RootMove> RootMoves;
47 Time::point SearchTime;
48 StateStackPtr SetupStates;
59 using namespace Search;
63 // Different node types, used as template parameter
64 enum NodeType { Root, PV, NonPV };
66 // Dynamic razoring margin based on depth
67 inline Value razor_margin(Depth d) { return Value(512 + 32 * d); }
69 // Futility lookup tables (initialized at startup) and their access functions
70 int FutilityMoveCounts[2][16]; // [improving][depth]
72 inline Value futility_margin(Depth d) {
73 return Value(200 * d);
76 // Reduction lookup tables (initialized at startup) and their access function
77 int8_t Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
79 template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
80 return (Depth) Reductions[PvNode][i][std::min(int(d), 63)][std::min(mn, 63)];
85 double BestMoveChanges;
86 Value DrawValue[COLOR_NB];
89 MovesStats Countermoves, Followupmoves;
91 template <NodeType NT, bool SpNode>
92 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
94 template <NodeType NT, bool InCheck>
95 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
97 void id_loop(Position& pos);
98 Value value_to_tt(Value v, int ply);
99 Value value_from_tt(Value v, int ply);
100 void update_pv(Move* pv, Move move, Move* childPv);
101 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
102 string uci_pv(const Position& pos, Depth depth, Value alpha, Value beta);
105 Skill(int l, size_t rootSize) : level(l),
106 candidates(l < 20 ? std::min(4, (int)rootSize) : 0),
109 if (candidates) // Swap best PV line with the sub-optimal one
110 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
111 RootMoves.end(), best ? best : pick_move()));
114 size_t candidates_size() const { return candidates; }
115 bool time_to_pick(Depth depth) const { return depth == 1 + level; }
126 /// Search::init() is called during startup to initialize various lookup tables
128 void Search::init() {
130 // Init reductions array
131 for (int d = 1; d < 64; ++d)
132 for (int mc = 1; mc < 64; ++mc)
134 double pvRed = 0.00 + log(double(d)) * log(double(mc)) / 3.00;
135 double nonPVRed = 0.33 + log(double(d)) * log(double(mc)) / 2.25;
137 Reductions[1][1][d][mc] = int8_t( pvRed >= 1.0 ? pvRed + 0.5: 0);
138 Reductions[0][1][d][mc] = int8_t(nonPVRed >= 1.0 ? nonPVRed + 0.5: 0);
140 Reductions[1][0][d][mc] = Reductions[1][1][d][mc];
141 Reductions[0][0][d][mc] = Reductions[0][1][d][mc];
143 // Increase reduction when eval is not improving
144 if (Reductions[0][0][d][mc] >= 2)
145 Reductions[0][0][d][mc] += 1;
148 // Init futility move count array
149 for (int d = 0; d < 16; ++d)
151 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
152 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
157 /// Search::perft() is our utility to verify move generation. All the leaf nodes
158 /// up to the given depth are generated and counted and the sum returned.
160 uint64_t Search::perft(Position& pos, Depth depth) {
163 uint64_t cnt, nodes = 0;
165 const bool leaf = (depth == 2 * ONE_PLY);
167 for (MoveList<LEGAL> it(pos); *it; ++it)
169 if (Root && depth <= ONE_PLY)
173 pos.do_move(*it, st, ci, pos.gives_check(*it, ci));
174 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
179 sync_cout << UCI::format_move(*it, pos.is_chess960()) << ": " << cnt << sync_endl;
184 template uint64_t Search::perft<true>(Position& pos, Depth depth);
187 /// Search::think() is the external interface to Stockfish's search, and is
188 /// called by the main thread when the program receives the UCI 'go' command. It
189 /// searches from RootPos and at the end prints the "bestmove" to output.
191 void Search::think() {
193 TimeMgr.init(Limits, RootPos.game_ply(), RootPos.side_to_move());
194 TBHits = TBCardinality = 0;
197 int cf = Options["Contempt"] * PawnValueEg / 100; // From centipawns
198 DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(cf);
199 DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(cf);
201 if (RootMoves.empty())
203 RootMoves.push_back(MOVE_NONE);
204 sync_cout << "info depth 0 score "
205 << UCI::format_value(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
211 // Check Tablebases at root
212 int piecesCnt = RootPos.total_piece_count();
213 TBCardinality = Options["SyzygyProbeLimit"];
214 TBProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
215 if (TBCardinality > Tablebases::TBLargest)
217 TBCardinality = Tablebases::TBLargest;
218 TBProbeDepth = 0 * ONE_PLY;
220 TB50MoveRule = Options["Syzygy50MoveRule"];
222 if (piecesCnt <= TBCardinality)
224 TBHits = RootMoves.size();
226 // If the current root position is in the tablebases then RootMoves
227 // contains only moves that preserve the draw or win.
228 RootInTB = Tablebases::root_probe(RootPos, TBScore);
232 TBCardinality = 0; // Do not probe tablebases during the search
234 // It might be a good idea to mangle the hash key (xor it
235 // with a fixed value) in order to "clear" the hash table of
236 // the results of previous probes. However, that would have to
237 // be done from within the Position class, so we skip it for now.
239 // Optional: decrease target time.
241 else // If DTZ tables are missing, use WDL tables as a fallback
243 // Filter out moves that do not preserve a draw or win
244 RootInTB = Tablebases::root_probe_wdl(RootPos, TBScore);
246 // Only probe during search if winning
247 if (TBScore <= VALUE_DRAW)
255 else if (!TB50MoveRule)
257 TBScore = TBScore > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
258 : TBScore < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
264 for (size_t i = 0; i < Threads.size(); ++i)
265 Threads[i]->maxPly = 0;
267 Threads.timer->run = true;
268 Threads.timer->notify_one(); // Wake up the recurring timer
270 id_loop(RootPos); // Let's start searching !
272 Threads.timer->run = false;
275 // When we reach the maximum depth, we can arrive here without a raise of
276 // Signals.stop. However, if we are pondering or in an infinite search,
277 // the UCI protocol states that we shouldn't print the best move before the
278 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
279 // until the GUI sends one of those commands (which also raises Signals.stop).
280 if (!Signals.stop && (Limits.ponder || Limits.infinite))
282 Signals.stopOnPonderhit = true;
283 RootPos.this_thread()->wait_for(Signals.stop);
286 sync_cout << "bestmove " << UCI::format_move(RootMoves[0].pv[0], RootPos.is_chess960());
288 if (RootMoves[0].pv.size() > 1)
289 std::cout << " ponder " << UCI::format_move(RootMoves[0].pv[1], RootPos.is_chess960());
291 std::cout << sync_endl;
297 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
298 // with increasing depth until the allocated thinking time has been consumed,
299 // user stops the search, or the maximum search depth is reached.
301 void id_loop(Position& pos) {
303 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
305 Value bestValue, alpha, beta, delta;
307 std::memset(ss-2, 0, 5 * sizeof(Stack));
311 bestValue = delta = alpha = -VALUE_INFINITE;
312 beta = VALUE_INFINITE;
317 Countermoves.clear();
318 Followupmoves.clear();
320 size_t multiPV = Options["MultiPV"];
321 Skill skill(Options["Skill Level"], RootMoves.size());
323 // Do we have to play with skill handicap? In this case enable MultiPV search
324 // that we will use behind the scenes to retrieve a set of possible moves.
325 multiPV = std::max(multiPV, skill.candidates_size());
327 // Iterative deepening loop until requested to stop or target depth reached
328 while (++depth < DEPTH_MAX && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
330 // Age out PV variability metric
331 BestMoveChanges *= 0.5;
333 // Save the last iteration's scores before first PV line is searched and
334 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
335 for (size_t i = 0; i < RootMoves.size(); ++i)
336 RootMoves[i].prevScore = RootMoves[i].score;
338 // MultiPV loop. We perform a full root search for each PV line
339 for (PVIdx = 0; PVIdx < std::min(multiPV, RootMoves.size()) && !Signals.stop; ++PVIdx)
341 // Reset aspiration window starting size
342 if (depth >= 5 * ONE_PLY)
345 alpha = std::max(RootMoves[PVIdx].prevScore - delta,-VALUE_INFINITE);
346 beta = std::min(RootMoves[PVIdx].prevScore + delta, VALUE_INFINITE);
349 // Start with a small aspiration window and, in the case of a fail
350 // high/low, re-search with a bigger window until we're not failing
354 bestValue = search<Root, false>(pos, ss, alpha, beta, depth, false);
356 // Bring the best move to the front. It is critical that sorting
357 // is done with a stable algorithm because all the values but the
358 // first and eventually the new best one are set to -VALUE_INFINITE
359 // and we want to keep the same order for all the moves except the
360 // new PV that goes to the front. Note that in case of MultiPV
361 // search the already searched PV lines are preserved.
362 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
364 // Write PV back to transposition table in case the relevant
365 // entries have been overwritten during the search.
366 for (size_t i = 0; i <= PVIdx; ++i)
367 RootMoves[i].insert_pv_in_tt(pos);
369 // If search has been stopped break immediately. Sorting and
370 // writing PV back to TT is safe because RootMoves is still
371 // valid, although it refers to previous iteration.
375 // When failing high/low give some update (without cluttering
376 // the UI) before a re-search.
377 if ( (bestValue <= alpha || bestValue >= beta)
378 && Time::now() - SearchTime > 3000)
379 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
381 // In case of failing low/high increase aspiration window and
382 // re-search, otherwise exit the loop.
383 if (bestValue <= alpha)
385 beta = (alpha + beta) / 2;
386 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
388 Signals.failedLowAtRoot = true;
389 Signals.stopOnPonderhit = false;
391 else if (bestValue >= beta)
393 alpha = (alpha + beta) / 2;
394 beta = std::min(bestValue + delta, VALUE_INFINITE);
401 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
404 // Sort the PV lines searched so far and update the GUI
405 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
408 sync_cout << "info nodes " << RootPos.nodes_searched()
409 << " time " << Time::now() - SearchTime << sync_endl;
411 else if ( PVIdx + 1 == std::min(multiPV, RootMoves.size())
412 || Time::now() - SearchTime > 3000)
413 sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
416 // If skill levels are enabled and time is up, pick a sub-optimal best move
417 if (skill.candidates_size() && skill.time_to_pick(depth))
420 // Have we found a "mate in x"?
422 && bestValue >= VALUE_MATE_IN_MAX_PLY
423 && VALUE_MATE - bestValue <= 2 * Limits.mate)
426 // Do we have time for the next iteration? Can we stop searching now?
427 if (Limits.use_time_management() && !Signals.stop && !Signals.stopOnPonderhit)
429 // Take some extra time if the best move has changed
430 if (depth > 4 * ONE_PLY && multiPV == 1)
431 TimeMgr.pv_instability(BestMoveChanges);
433 // Stop the search if only one legal move is available or all
434 // of the available time has been used.
435 if ( RootMoves.size() == 1
436 || Time::now() - SearchTime > TimeMgr.available_time())
438 // If we are allowed to ponder do not stop the search now but
439 // keep pondering until the GUI sends "ponderhit" or "stop".
441 Signals.stopOnPonderhit = true;
450 // search<>() is the main search function for both PV and non-PV nodes and for
451 // normal and SplitPoint nodes. When called just after a split point the search
452 // is simpler because we have already probed the hash table, done a null move
453 // search, and searched the first move before splitting, so we don't have to
454 // repeat all this work again. We also don't need to store anything to the hash
455 // table here: This is taken care of after we return from the split point.
457 template <NodeType NT, bool SpNode>
458 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
460 const bool RootNode = NT == Root;
461 const bool PvNode = NT == PV || NT == Root;
463 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
464 assert(PvNode || (alpha == beta - 1));
465 assert(depth > DEPTH_ZERO);
467 Move pv[MAX_PLY+1], quietsSearched[64];
470 SplitPoint* splitPoint;
472 Move ttMove, move, excludedMove, bestMove;
473 Depth ext, newDepth, predictedDepth;
474 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
475 bool inCheck, givesCheck, singularExtensionNode, improving;
476 bool captureOrPromotion, dangerous, doFullDepthSearch;
477 int moveCount, quietCount;
479 // Step 1. Initialize node
480 Thread* thisThread = pos.this_thread();
481 inCheck = pos.checkers();
485 splitPoint = ss->splitPoint;
486 bestMove = splitPoint->bestMove;
487 bestValue = splitPoint->bestValue;
489 ttMove = excludedMove = MOVE_NONE;
490 ttValue = VALUE_NONE;
492 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
497 moveCount = quietCount = 0;
498 bestValue = -VALUE_INFINITE;
499 ss->ply = (ss-1)->ply + 1;
501 // Used to send selDepth info to GUI
502 if (PvNode && thisThread->maxPly < ss->ply)
503 thisThread->maxPly = ss->ply;
507 // Step 2. Check for aborted search and immediate draw
508 if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
509 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
511 // Step 3. Mate distance pruning. Even if we mate at the next move our score
512 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
513 // a shorter mate was found upward in the tree then there is no need to search
514 // because we will never beat the current alpha. Same logic but with reversed
515 // signs applies also in the opposite condition of being mated instead of giving
516 // mate. In this case return a fail-high score.
517 alpha = std::max(mated_in(ss->ply), alpha);
518 beta = std::min(mate_in(ss->ply+1), beta);
523 assert(0 <= ss->ply && ss->ply < MAX_PLY);
525 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
526 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
527 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
529 // Step 4. Transposition table lookup
530 // We don't want the score of a partial search to overwrite a previous full search
531 // TT value, so we use a different position key in case of an excluded move.
532 excludedMove = ss->excludedMove;
533 posKey = excludedMove ? pos.exclusion_key() : pos.key();
534 tte = TT.probe(posKey);
535 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
536 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
538 // At non-PV nodes we check for a fail high/low. We don't probe at PV nodes
541 && tte->depth() >= depth
542 && ttValue != VALUE_NONE // Only in case of TT access race
543 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
544 : (tte->bound() & BOUND_UPPER)))
546 ss->currentMove = ttMove; // Can be MOVE_NONE
548 // If ttMove is quiet, update killers, history, counter move and followup move on TT hit
549 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove) && !inCheck)
550 update_stats(pos, ss, ttMove, depth, NULL, 0);
556 // Step 4a. Tablebase probe
558 && pos.total_piece_count() <= TBCardinality
559 && ( pos.total_piece_count() < TBCardinality || depth >= TBProbeDepth )
560 && pos.rule50_count() == 0)
562 int found, v = Tablebases::probe_wdl(pos, &found);
569 value = v < -1 ? -VALUE_MATE + MAX_PLY + ss->ply
570 : v > 1 ? VALUE_MATE - MAX_PLY - ss->ply
571 : VALUE_DRAW + 2 * v;
575 value = v < 0 ? -VALUE_MATE + MAX_PLY + ss->ply
576 : v > 0 ? VALUE_MATE - MAX_PLY - ss->ply
580 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
581 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY), MOVE_NONE, VALUE_NONE);
588 // Step 5. Evaluate the position statically and update parent's gain statistics
591 ss->staticEval = eval = VALUE_NONE;
597 // Never assume anything on values stored in TT
598 if ((ss->staticEval = eval = tte->eval_value()) == VALUE_NONE)
599 eval = ss->staticEval = evaluate(pos);
601 // Can ttValue be used as a better position evaluation?
602 if (ttValue != VALUE_NONE)
603 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
608 eval = ss->staticEval =
609 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
611 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval);
614 if ( !pos.captured_piece_type()
615 && ss->staticEval != VALUE_NONE
616 && (ss-1)->staticEval != VALUE_NONE
617 && (move = (ss-1)->currentMove) != MOVE_NULL
619 && type_of(move) == NORMAL)
621 Square to = to_sq(move);
622 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
625 // Step 6. Razoring (skipped when in check)
627 && depth < 4 * ONE_PLY
628 && eval + razor_margin(depth) <= alpha
629 && ttMove == MOVE_NONE
630 && !pos.pawn_on_7th(pos.side_to_move()))
632 if ( depth <= ONE_PLY
633 && eval + razor_margin(3 * ONE_PLY) <= alpha)
634 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
636 Value ralpha = alpha - razor_margin(depth);
637 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
642 // Step 7. Futility pruning: child node (skipped when in check)
645 && depth < 7 * ONE_PLY
646 && eval - futility_margin(depth) >= beta
647 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
648 && pos.non_pawn_material(pos.side_to_move()))
649 return eval - futility_margin(depth);
651 // Step 8. Null move search with verification search (is omitted in PV nodes)
654 && depth >= 2 * ONE_PLY
656 && pos.non_pawn_material(pos.side_to_move()))
658 ss->currentMove = MOVE_NULL;
660 assert(eval - beta >= 0);
662 // Null move dynamic reduction based on depth and value
663 Depth R = (3 + depth / 4 + std::min(int(eval - beta) / PawnValueMg, 3)) * ONE_PLY;
665 pos.do_null_move(st);
666 (ss+1)->skipNullMove = true;
667 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
668 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
669 (ss+1)->skipNullMove = false;
670 pos.undo_null_move();
672 if (nullValue >= beta)
674 // Do not return unproven mate scores
675 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
678 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
681 // Do verification search at high depths
682 ss->skipNullMove = true;
683 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
684 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
685 ss->skipNullMove = false;
692 // Step 9. ProbCut (skipped when in check)
693 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
694 // and a reduced search returns a value much above beta, we can (almost) safely
695 // prune the previous move.
697 && depth >= 5 * ONE_PLY
699 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
701 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
702 Depth rdepth = depth - 4 * ONE_PLY;
704 assert(rdepth >= ONE_PLY);
705 assert((ss-1)->currentMove != MOVE_NONE);
706 assert((ss-1)->currentMove != MOVE_NULL);
708 MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
711 while ((move = mp.next_move<false>()) != MOVE_NONE)
712 if (pos.legal(move, ci.pinned))
714 ss->currentMove = move;
715 pos.do_move(move, st, ci, pos.gives_check(move, ci));
716 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
723 // Step 10. Internal iterative deepening (skipped when in check)
724 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
726 && (PvNode || ss->staticEval + 256 >= beta))
728 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
729 ss->skipNullMove = true;
730 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
731 ss->skipNullMove = false;
733 tte = TT.probe(posKey);
734 ttMove = tte ? tte->move() : MOVE_NONE;
737 moves_loop: // When in check and at SpNode search starts from here
739 Square prevMoveSq = to_sq((ss-1)->currentMove);
740 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
741 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
743 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
744 Move followupmoves[] = { Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].first,
745 Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].second };
747 MovePicker mp(pos, ttMove, depth, History, countermoves, followupmoves, ss);
749 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
750 improving = ss->staticEval >= (ss-2)->staticEval
751 || ss->staticEval == VALUE_NONE
752 ||(ss-2)->staticEval == VALUE_NONE;
754 singularExtensionNode = !RootNode
756 && depth >= 8 * ONE_PLY
757 && ttMove != MOVE_NONE
758 /* && ttValue != VALUE_NONE Already implicit in the next condition */
759 && abs(ttValue) < VALUE_KNOWN_WIN
760 && !excludedMove // Recursive singular search is not allowed
761 && (tte->bound() & BOUND_LOWER)
762 && tte->depth() >= depth - 3 * ONE_PLY;
764 // Step 11. Loop through moves
765 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
766 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
770 if (move == excludedMove)
773 // At root obey the "searchmoves" option and skip moves not listed in Root
774 // Move List. As a consequence any illegal move is also skipped. In MultiPV
775 // mode we also skip PV moves which have been already searched.
776 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
781 // Shared counter cannot be decremented later if the move turns out to be illegal
782 if (!pos.legal(move, ci.pinned))
785 moveCount = ++splitPoint->moveCount;
786 splitPoint->mutex.unlock();
793 Signals.firstRootMove = (moveCount == 1);
795 if (thisThread == Threads.main() && Time::now() - SearchTime > 3000)
796 sync_cout << "info depth " << depth
797 << " currmove " << UCI::format_move(move, pos.is_chess960())
798 << " currmovenumber " << moveCount + PVIdx << sync_endl;
805 captureOrPromotion = pos.capture_or_promotion(move);
807 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
808 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
809 : pos.gives_check(move, ci);
811 dangerous = givesCheck
812 || type_of(move) != NORMAL
813 || pos.advanced_pawn_push(move);
815 // Step 12. Extend checks
816 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
819 // Singular extension search. If all moves but one fail low on a search of
820 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
821 // is singular and should be extended. To verify this we do a reduced search
822 // on all the other moves but the ttMove and if the result is lower than
823 // ttValue minus a margin then we extend the ttMove.
824 if ( singularExtensionNode
827 && pos.legal(move, ci.pinned))
829 Value rBeta = ttValue - int(2 * depth);
830 ss->excludedMove = move;
831 ss->skipNullMove = true;
832 value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
833 ss->skipNullMove = false;
834 ss->excludedMove = MOVE_NONE;
840 // Update the current move (this must be done after singular extension search)
841 newDepth = depth - ONE_PLY + ext;
843 // Step 13. Pruning at shallow depth (exclude PV nodes)
845 && !captureOrPromotion
848 && bestValue > VALUE_MATED_IN_MAX_PLY)
850 // Move count based pruning
851 if ( depth < 16 * ONE_PLY
852 && moveCount >= FutilityMoveCounts[improving][depth])
855 splitPoint->mutex.lock();
860 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
862 // Futility pruning: parent node
863 if (predictedDepth < 7 * ONE_PLY)
865 futilityValue = ss->staticEval + futility_margin(predictedDepth)
866 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
868 if (futilityValue <= alpha)
870 bestValue = std::max(bestValue, futilityValue);
874 splitPoint->mutex.lock();
875 if (bestValue > splitPoint->bestValue)
876 splitPoint->bestValue = bestValue;
882 // Prune moves with negative SEE at low depths
883 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
886 splitPoint->mutex.lock();
892 // Speculative prefetch as early as possible
893 prefetch((char*)TT.first_entry(pos.key_after(move)));
895 // Check for legality just before making the move
896 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
902 ss->currentMove = move;
903 if (!SpNode && !captureOrPromotion && quietCount < 64)
904 quietsSearched[quietCount++] = move;
906 // Step 14. Make the move
907 pos.do_move(move, st, ci, givesCheck);
909 // Step 15. Reduced depth search (LMR). If the move fails high it will be
910 // re-searched at full depth.
911 if ( depth >= 3 * ONE_PLY
913 && !captureOrPromotion
914 && move != ss->killers[0]
915 && move != ss->killers[1])
917 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
919 if ( (!PvNode && cutNode)
920 || History[pos.piece_on(to_sq(move))][to_sq(move)] < 0)
921 ss->reduction += ONE_PLY;
923 if (move == countermoves[0] || move == countermoves[1])
924 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
926 // Decrease reduction for moves that escape a capture
928 && type_of(move) == NORMAL
929 && type_of(pos.piece_on(to_sq(move))) != PAWN
930 && pos.see(make_move(to_sq(move), from_sq(move))) < 0)
931 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
933 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
935 alpha = splitPoint->alpha;
937 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
939 // Re-search at intermediate depth if reduction is very high
940 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
942 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
943 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
946 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
947 ss->reduction = DEPTH_ZERO;
950 doFullDepthSearch = !PvNode || moveCount > 1;
952 // Step 16. Full depth search, when LMR is skipped or fails high
953 if (doFullDepthSearch)
956 alpha = splitPoint->alpha;
958 value = newDepth < ONE_PLY ?
959 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
960 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
961 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
964 // For PV nodes only, do a full PV search on the first move or after a fail
965 // high (in the latter case search only if value < beta), otherwise let the
966 // parent node fail low with value <= alpha and to try another move.
967 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
970 (ss+1)->pv[0] = MOVE_NONE;
972 value = newDepth < ONE_PLY ?
973 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
974 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
975 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
978 // Step 17. Undo move
981 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
983 // Step 18. Check for new best move
986 splitPoint->mutex.lock();
987 bestValue = splitPoint->bestValue;
988 alpha = splitPoint->alpha;
991 // Finished searching the move. If a stop or a cutoff occurred, the return
992 // value of the search cannot be trusted, and we return immediately without
993 // updating best move, PV and TT.
994 if (Signals.stop || thisThread->cutoff_occurred())
999 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1001 // PV move or new best move ?
1002 if (moveCount == 1 || value > alpha)
1009 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1010 rm.pv.push_back(*m);
1012 // We record how often the best move has been changed in each
1013 // iteration. This information is used for time management: When
1014 // the best move changes frequently, we allocate some more time.
1019 // All other moves but the PV are set to the lowest value: this is
1020 // not a problem when sorting because the sort is stable and the
1021 // move position in the list is preserved - just the PV is pushed up.
1022 rm.score = -VALUE_INFINITE;
1025 if (value > bestValue)
1027 bestValue = SpNode ? splitPoint->bestValue = value : value;
1031 bestMove = SpNode ? splitPoint->bestMove = move : move;
1033 if (PvNode && !RootNode) // Update pv even in fail-high case
1034 update_pv(SpNode ? splitPoint->ss->pv : ss->pv, move, (ss+1)->pv);
1036 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1037 alpha = SpNode ? splitPoint->alpha = value : value;
1040 assert(value >= beta); // Fail high
1043 splitPoint->cutoff = true;
1050 // Step 19. Check for splitting the search
1052 && Threads.size() >= 2
1053 && depth >= Threads.minimumSplitDepth
1054 && ( !thisThread->activeSplitPoint
1055 || !thisThread->activeSplitPoint->allSlavesSearching)
1056 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1058 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
1060 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
1061 depth, moveCount, &mp, NT, cutNode);
1063 if (Signals.stop || thisThread->cutoff_occurred())
1066 if (bestValue >= beta)
1074 // Following condition would detect a stop or a cutoff set only after move
1075 // loop has been completed. But in this case bestValue is valid because we
1076 // have fully searched our subtree, and we can anyhow save the result in TT.
1078 if (Signals.stop || thisThread->cutoff_occurred())
1082 // Step 20. Check for mate and stalemate
1083 // All legal moves have been searched and if there are no legal moves, it
1084 // must be mate or stalemate. If we are in a singular extension search then
1085 // return a fail low score.
1087 bestValue = excludedMove ? alpha
1088 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1090 // Quiet best move: update killers, history, countermoves and followupmoves
1091 else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
1092 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
1094 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1095 bestValue >= beta ? BOUND_LOWER :
1096 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1097 depth, bestMove, ss->staticEval);
1099 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1105 // qsearch() is the quiescence search function, which is called by the main
1106 // search function when the remaining depth is zero (or, to be more precise,
1107 // less than ONE_PLY).
1109 template <NodeType NT, bool InCheck>
1110 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1112 const bool PvNode = NT == PV;
1114 assert(NT == PV || NT == NonPV);
1115 assert(InCheck == !!pos.checkers());
1116 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1117 assert(PvNode || (alpha == beta - 1));
1118 assert(depth <= DEPTH_ZERO);
1124 Move ttMove, move, bestMove;
1125 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1126 bool givesCheck, evasionPrunable;
1131 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1133 ss->pv[0] = MOVE_NONE;
1136 ss->currentMove = bestMove = MOVE_NONE;
1137 ss->ply = (ss-1)->ply + 1;
1139 // Check for an instant draw or if the maximum ply has been reached
1140 if (pos.is_draw() || ss->ply >= MAX_PLY)
1141 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1143 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1145 // Decide whether or not to include checks: this fixes also the type of
1146 // TT entry depth that we are going to use. Note that in qsearch we use
1147 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1148 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1149 : DEPTH_QS_NO_CHECKS;
1151 // Transposition table lookup
1153 tte = TT.probe(posKey);
1154 ttMove = tte ? tte->move() : MOVE_NONE;
1155 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
1159 && tte->depth() >= ttDepth
1160 && ttValue != VALUE_NONE // Only in case of TT access race
1161 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1162 : (tte->bound() & BOUND_UPPER)))
1164 ss->currentMove = ttMove; // Can be MOVE_NONE
1168 // Evaluate the position statically
1171 ss->staticEval = VALUE_NONE;
1172 bestValue = futilityBase = -VALUE_INFINITE;
1178 // Never assume anything on values stored in TT
1179 if ((ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE)
1180 ss->staticEval = bestValue = evaluate(pos);
1182 // Can ttValue be used as a better position evaluation?
1183 if (ttValue != VALUE_NONE)
1184 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1185 bestValue = ttValue;
1188 ss->staticEval = bestValue =
1189 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1191 // Stand pat. Return immediately if static value is at least beta
1192 if (bestValue >= beta)
1195 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1196 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1201 if (PvNode && bestValue > alpha)
1204 futilityBase = bestValue + 128;
1207 // Initialize a MovePicker object for the current position, and prepare
1208 // to search the moves. Because the depth is <= 0 here, only captures,
1209 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1211 MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
1214 // Loop through the moves until no moves remain or a beta cutoff occurs
1215 while ((move = mp.next_move<false>()) != MOVE_NONE)
1217 assert(is_ok(move));
1219 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1220 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1221 : pos.gives_check(move, ci);
1227 && futilityBase > -VALUE_KNOWN_WIN
1228 && !pos.advanced_pawn_push(move))
1230 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1232 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1234 if (futilityValue < beta)
1236 bestValue = std::max(bestValue, futilityValue);
1240 if (futilityBase < beta && pos.see(move) <= VALUE_ZERO)
1242 bestValue = std::max(bestValue, futilityBase);
1247 // Detect non-capture evasions that are candidates to be pruned
1248 evasionPrunable = InCheck
1249 && bestValue > VALUE_MATED_IN_MAX_PLY
1250 && !pos.capture(move)
1251 && !pos.can_castle(pos.side_to_move());
1253 // Don't search moves with negative SEE values
1255 && (!InCheck || evasionPrunable)
1256 && type_of(move) != PROMOTION
1257 && pos.see_sign(move) < VALUE_ZERO)
1260 // Speculative prefetch as early as possible
1261 prefetch((char*)TT.first_entry(pos.key_after(move)));
1263 // Check for legality just before making the move
1264 if (!pos.legal(move, ci.pinned))
1267 ss->currentMove = move;
1269 // Make and search the move
1270 pos.do_move(move, st, ci, givesCheck);
1271 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1272 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1273 pos.undo_move(move);
1275 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1277 // Check for new best move
1278 if (value > bestValue)
1284 if (PvNode) // Update pv even in fail-high case
1285 update_pv(ss->pv, move, (ss+1)->pv);
1287 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1294 TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1295 ttDepth, move, ss->staticEval);
1303 // All legal moves have been searched. A special case: If we're in check
1304 // and no legal moves were found, it is checkmate.
1305 if (InCheck && bestValue == -VALUE_INFINITE)
1306 return mated_in(ss->ply); // Plies to mate from the root
1308 TT.store(posKey, value_to_tt(bestValue, ss->ply),
1309 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1310 ttDepth, bestMove, ss->staticEval);
1312 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1318 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1319 // "plies to mate from the current position". Non-mate scores are unchanged.
1320 // The function is called before storing a value in the transposition table.
1322 Value value_to_tt(Value v, int ply) {
1324 assert(v != VALUE_NONE);
1326 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1327 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1331 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1332 // from the transposition table (which refers to the plies to mate/be mated
1333 // from current position) to "plies to mate/be mated from the root".
1335 Value value_from_tt(Value v, int ply) {
1337 return v == VALUE_NONE ? VALUE_NONE
1338 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1339 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1343 // update_pv() adds current move and appends child pv[]
1345 void update_pv(Move* pv, Move move, Move* childPv) {
1347 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1352 // update_stats() updates killers, history, countermoves and followupmoves stats after a fail-high
1355 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt) {
1357 if (ss->killers[0] != move)
1359 ss->killers[1] = ss->killers[0];
1360 ss->killers[0] = move;
1363 // Increase history value of the cut-off move and decrease all the other
1364 // played quiet moves.
1365 Value bonus = Value(int(depth) * int(depth));
1366 History.update(pos.moved_piece(move), to_sq(move), bonus);
1367 for (int i = 0; i < quietsCnt; ++i)
1370 History.update(pos.moved_piece(m), to_sq(m), -bonus);
1373 if (is_ok((ss-1)->currentMove))
1375 Square prevMoveSq = to_sq((ss-1)->currentMove);
1376 Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, move);
1379 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1381 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
1382 Followupmoves.update(pos.piece_on(prevOwnMoveSq), prevOwnMoveSq, move);
1387 // When playing with a strength handicap, choose best move among the first 'candidates'
1388 // RootMoves using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1390 Move Skill::pick_move() {
1394 // PRNG sequence should be not deterministic
1395 for (int i = Time::now() % 50; i > 0; --i)
1396 rk.rand<unsigned>();
1398 // RootMoves are already sorted by score in descending order
1399 int variance = std::min(RootMoves[0].score - RootMoves[candidates - 1].score, PawnValueMg);
1400 int weakness = 120 - 2 * level;
1401 int max_s = -VALUE_INFINITE;
1404 // Choose best move. For each move score we add two terms both dependent on
1405 // weakness. One deterministic and bigger for weaker moves, and one random,
1406 // then we choose the move with the resulting highest score.
1407 for (size_t i = 0; i < candidates; ++i)
1409 int s = RootMoves[i].score;
1411 // Don't allow crazy blunders even at very low skills
1412 if (i > 0 && RootMoves[i - 1].score > s + 2 * PawnValueMg)
1415 // This is our magic formula
1416 s += ( weakness * int(RootMoves[0].score - s)
1417 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1422 best = RootMoves[i].pv[0];
1429 // uci_pv() formats PV information according to the UCI protocol. UCI
1430 // requires that all (if any) unsearched PV lines are sent using a previous
1433 string uci_pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1435 std::stringstream ss;
1436 Time::point elapsed = Time::now() - SearchTime + 1;
1437 size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
1440 for (size_t i = 0; i < Threads.size(); ++i)
1441 if (Threads[i]->maxPly > selDepth)
1442 selDepth = Threads[i]->maxPly;
1444 for (size_t i = 0; i < uciPVSize; ++i)
1446 bool updated = (i <= PVIdx);
1448 if (depth == 1 && !updated)
1451 Depth d = updated ? depth : depth - ONE_PLY;
1452 Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
1457 if (abs(v) >= VALUE_MATE - MAX_PLY)
1463 if (ss.rdbuf()->in_avail()) // Not at first line
1466 ss << "info depth " << d / ONE_PLY
1467 << " seldepth " << selDepth
1468 << " multipv " << i + 1
1469 << " score " << ((!tb && i == PVIdx) ? UCI::format_value(v, alpha, beta) : UCI::format_value(v))
1470 << " nodes " << pos.nodes_searched()
1471 << " nps " << pos.nodes_searched() * 1000 / elapsed
1472 << " tbhits " << TBHits
1473 << " time " << elapsed
1476 for (size_t j = 0; j < RootMoves[i].pv.size(); ++j)
1477 ss << " " << UCI::format_move(RootMoves[i].pv[j], pos.is_chess960());
1486 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1487 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1488 /// first, even if the old TT entries have been overwritten.
1490 void RootMove::insert_pv_in_tt(Position& pos) {
1492 StateInfo state[MAX_PLY], *st = state;
1496 for ( ; idx < pv.size(); ++idx)
1498 tte = TT.probe(pos.key());
1500 if (!tte || tte->move() != pv[idx]) // Don't overwrite correct entries
1501 TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[idx], VALUE_NONE);
1503 assert(MoveList<LEGAL>(pos).contains(pv[idx]));
1505 pos.do_move(pv[idx], *st++);
1508 while (idx) pos.undo_move(pv[--idx]);
1512 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1514 void Thread::idle_loop() {
1516 // Pointer 'this_sp' is not null only if we are called from split(), and not
1517 // at the thread creation. This means we are the split point's master.
1518 SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
1520 assert(!this_sp || (this_sp->masterThread == this && searching));
1524 // If this thread has been assigned work, launch a search
1527 Threads.mutex.lock();
1529 assert(activeSplitPoint);
1530 SplitPoint* sp = activeSplitPoint;
1532 Threads.mutex.unlock();
1534 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
1535 Position pos(*sp->pos, this);
1537 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1538 ss->splitPoint = sp;
1542 assert(activePosition == NULL);
1544 activePosition = &pos;
1546 if (sp->nodeType == NonPV)
1547 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1549 else if (sp->nodeType == PV)
1550 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1552 else if (sp->nodeType == Root)
1553 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1561 activePosition = NULL;
1562 sp->slavesMask.reset(idx);
1563 sp->allSlavesSearching = false;
1564 sp->nodes += pos.nodes_searched();
1566 // Wake up the master thread so to allow it to return from the idle
1567 // loop in case we are the last slave of the split point.
1568 if ( this != sp->masterThread
1569 && sp->slavesMask.none())
1571 assert(!sp->masterThread->searching);
1572 sp->masterThread->notify_one();
1575 // After releasing the lock we can't access any SplitPoint related data
1576 // in a safe way because it could have been released under our feet by
1580 // Try to late join to another split point if none of its slaves has
1581 // already finished.
1582 if (Threads.size() > 2)
1583 for (size_t i = 0; i < Threads.size(); ++i)
1585 const int size = Threads[i]->splitPointsSize; // Local copy
1586 sp = size ? &Threads[i]->splitPoints[size - 1] : NULL;
1589 && sp->allSlavesSearching
1590 && available_to(Threads[i]))
1592 // Recheck the conditions under lock protection
1593 Threads.mutex.lock();
1596 if ( sp->allSlavesSearching
1597 && available_to(Threads[i]))
1599 sp->slavesMask.set(idx);
1600 activeSplitPoint = sp;
1605 Threads.mutex.unlock();
1607 break; // Just a single attempt
1612 // Grab the lock to avoid races with Thread::notify_one()
1615 // If we are master and all slaves have finished then exit idle_loop
1616 if (this_sp && this_sp->slavesMask.none())
1623 // If we are not searching, wait for a condition to be signaled instead of
1624 // wasting CPU time polling for work.
1625 if (!searching && !exit)
1626 sleepCondition.wait(mutex);
1633 /// check_time() is called by the timer thread when the timer triggers. It is
1634 /// used to print debug info and, more importantly, to detect when we are out of
1635 /// available time and thus stop the search.
1639 static Time::point lastInfoTime = Time::now();
1640 Time::point elapsed = Time::now() - SearchTime;
1642 if (Time::now() - lastInfoTime >= 1000)
1644 lastInfoTime = Time::now();
1648 // An engine may not stop pondering until told so by the GUI
1652 if (Limits.use_time_management())
1654 bool stillAtFirstMove = Signals.firstRootMove
1655 && !Signals.failedLowAtRoot
1656 && elapsed > TimeMgr.available_time() * 75 / 100;
1658 if ( stillAtFirstMove
1659 || elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution)
1660 Signals.stop = true;
1662 else if (Limits.movetime && elapsed >= Limits.movetime)
1663 Signals.stop = true;
1665 else if (Limits.nodes)
1667 Threads.mutex.lock();
1669 int64_t nodes = RootPos.nodes_searched();
1671 // Loop across all split points and sum accumulated SplitPoint nodes plus
1672 // all the currently active positions nodes.
1673 for (size_t i = 0; i < Threads.size(); ++i)
1674 for (int j = 0; j < Threads[i]->splitPointsSize; ++j)
1676 SplitPoint& sp = Threads[i]->splitPoints[j];
1682 for (size_t idx = 0; idx < Threads.size(); ++idx)
1683 if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
1684 nodes += Threads[idx]->activePosition->nodes_searched();
1689 Threads.mutex.unlock();
1691 if (nodes >= Limits.nodes)
1692 Signals.stop = true;